Nowadays, mankind's activities affect our environment on a local, regional and global scale. This has led to serious environmental disturbances. Examples related to our atmosphere are concentrations of air pollutants in urban air and industrial areas which are injurious to heath, the regional formation of ozone and the destruction of trees by pollution, and global effects on the climate and the protective ozone layer in the stratosphere. In order to understand the mechanisms behind these problems and to be able to take cost-effective action, there is a requirement for measurements of the gas emissions which are the cause of the problems which have arisen. In some cases, these emissions are well-demarcated locally, for example discharges through chimneys and vents, and the emissions can be measured by measuring the concentration and gas flow at the source. In many cases, however, the emission source is “diffuse”, i.e. dispersed over a relatively large area. Examples of this are the emission of hydrocarbons from petrochemical plants or a refinery, the emission of methane from a refuse disposal site or the emission of nitrogen oxides from a town. Quantifying the emission from this type of source constitutes a substantially greater measurement challenge from the technical point of view. Five different measurement strategies which are nowadays used for quantifying diffuse gaseous emissions are described in the following section.
1. Meteorological Method:
In this method, the concentration of the gas in question is measured at one or several points at a suitable distance from the source. By means of using a meteorological dispersion model, the dispersion of the gas can be modelled and the strength of the source can be calculated. The method depends entirely on the reliability of the meteorological model and can therefore only be used when the topography and the meteorology are suitable. An example is the measurement of emissions such as carbon dioxide and methane from arable land (flat terrain, no buildings or trees allowed to disturb the wind field within 200-500 m). When industrial emissions are being measured, the method is quite unsuitable due to the effect of the buildings on the wind field, hot discharges, etc.
2. The Tracer Gas Method:
This can be said to be a variant of the above-mentioned method. In this case, too, the concentration of the gas under study is measured at one or several points on the lee side of the source. Instead of calculating the dispersal using a meteorological model, this is done empirically by releasing a tracer gas of known emission at the source. The concentration of the tracer gas is then measured at the same time as the gas under study is measured. In this connection, it is possible to determine the relationship between the concentration and emission in the case of the tracer gas. If the tracer gas has the same physical properties (temperature and density) as the gas under study, this relationship can then also be used for determining the emission of the gas under study. The method presupposes that the two gases are mixed well, something which in turn presupposes suitable meteorological conditions and that the tracer gas is released in a manner which simulates well the emission of the gas under study.
3. Line Integration:
Both the above-mentioned methods suffer from the weakness that the measurements are only made at one or several points in the air mass which is emitted. The reliability of the determinations can be substantially improved by using line integration to make the concentration measurements. This can be achieved by using line-integrating optical methods such as DOAS (differential optical absorption spectroscopy) or LPFTIR (long-path Fourier transform infrared spectroscopy), what is termed “cross-wind integration”. In this connection, electromagnetic radiation is transmitted over a distance which transects the air mass in question and absorption spectroscopy can be used to determine the mean concentration over the measurement distance in question, after which the emission is determined using one of the above-mentioned methods.
4. Laser Radar:
Additional reliability in the determination can be obtained if the gas concentration is integrated over the whole of the cross section of the emitted gas mass on the lee side of the source. A method which does this is DIAL (differential absorption lidar). This method is used to emit laser light of different wavelengths in a well-defined direction. By means of the time-resolved detection of the light which is backscattered from particles and molecules, it is possible to obtain a distance-resolved determination of the gas concentration along the laser beam. By measuring sequentially in different directions using different elevations, it is possible to obtain the integrated concentration over a cross section of the whole of the emitted gas mass. If the cross section is placed perpendicular to the wind, the total emission can then be obtained if the concentration is multiplied by the concentration-weighted wind speed. The strength of the method is that the whole of the emitted gas mass is integrated and that it is not necessary to use a meteorological dispersal model. The uncertainty in the measurements is largely determined by the uncertainty in determining the wind field. The disadvantages of the method are that only a few gases can be measured and that the measuring equipment is expensive and complicated and requires qualified personnel, thereby making the measurements expensive.
5. Sky-light Spectroscopy:
Integrating sky-light spectroscopy is an alternative method which also integrates over the whole of the cross section of the emitted air mass. This method uses a spectrometer to record the light of the zenithal sky. This results in a spectrum of the zenithal sky including the vertically integrated concentration of the gases which are present in the atmosphere. By means of moving the spectrometer in such a way that the vertical stretch which is measured cuts the emitted gas mass, it is possible, after subtracting the contribution made by the background concentration, to determine the integrated concentration over a cross section of the emitted gas mass. The emission is obtained after multiplying by the concentration-weighted wind speed transversely to the direction of traversal. While the method uses relatively simple equipment, it is limited to a small number of molecules which can be measured in the 300-700 nm range of the electromagnetic spectrum, i.e. the spectral range within which the zenithal sky scatters sunlight. In addition, the measurements are affected by variable multiple scattering, for example in clouds, and by what is termed the Ring effect (partial filling-in of the Fraunhofer lines in the solar spectrum caused by Raman scattering). The method has therefore principally been used for measuring industrial emissions of SO2 and NO2 and for quantifying emissions of SO2 from volcanoes.